In recent years, various sorts of semiconductor lasers are widely generally used as light sources for optical disk devices. Among others, a high-output-power semiconductor laser is used as a light source for write into disks of an MD drive, an MO drive, a CD-R/RW drive, a writable DVD drive or the like and is demanded to have a higher output power.
One factor of hindering the increase in output power of the semiconductor laser is an optical damage (COD: Catastrophic Optical Damage) that occurs with an increase in the optical output density in an active layer region located in the vicinity of a resonator end surface.
This COD occurs for the reason that the active layer region located in the vicinity of the resonator end surface of the active layer is a laser light absorbing region. A great number of nonluminescence recombination centers, which is called the surface state or the interface state, exist on the resonator end surface. Carriers injected into a region in the vicinity of the resonator end surface of the active layer are lost by this nonluminescence recombination, and therefore, the injection carrier density in the vicinity of the resonator end surface of the active layer is less than in the internal region of the active layer. As a result, the region located in the vicinity of the resonator end surface of the active layer becomes an absorption region with respect to the wavelength of the laser light formed by the carrier of high injection density in the internal region of the active layer.
When the optical output density is increased, local heating in the absorption region becomes increased, the temperature rises and the bandgap is reduced. As a result, a positive feedback that the absorption coefficient becomes further increased and the temperature rises takes effect, and the temperature of the absorption region formed in the vicinity of the resonator end surface of the active layer finally reaches the melting point, causing the COD.
For the improvement of this COD level, a method for utilizing a window structure based on the disorder of a multiple quantum well structure active layer is disclosed as a method for increasing the output power of the semiconductor laser in Japanese Patent Laid-Open Publication No. HEI 9-23037. This semiconductor laser element will be described with reference to FIGS. 17A through 17C, which are structural views thereof, and FIGS. 18A through 18D, which are process charts of the fabricating method thereof.
FIG. 17A is a perspective view including the resonator end surface. FIG. 17B is a sectional view taken along the line Ib—Ib in FIG. 17A. FIG. 17C is a sectional view in the direction of layer thickness taken along the line Ic—Ic in FIG. 17A.
FIGS. 17A through 17C show a GaAs substrate 1001, an n-type AlGaAs lower clad layer 1002, a quantum well active layer 1003, a p-type AlGaAs upper clad layer 1004a, a p-type AlGaAs second upper clad layer 1004b, a p-type GaAs contact layer 1005, a hole diffusion region (hatched region) 1006, a proton implantation region (hatched region) 1007, an n-side electrode 1008, a p-side electrode 1009, a resonator end surface 1020, a region (hereinafter referred to as an internal region) 1003a that contributes to the laser oscillation of the quantum well active layer 1003 and a window structure region 1003b formed in the vicinity of the resonator end surface 1020 of the quantum well active layer 1003.
In FIGS. 18A through 18D, the n-type AlGaAs lower clad layer 1002, the quantum well active layer 1003 and a p-type AlGaAs first upper clad layer 1004a are successively epitaxially grown on the n-type GaAs substrate 1001 (FIG. 18A). Next, an SiO2 film 1010 is formed on the p-type AlGaAs first upper clad layer 1004a, and a stripe-shaped opening 1010a that extends in the direction of the resonator is formed with a length that does not reach the resonator end surface (FIG. 18B). Next, when this wafer is subjected to heat treatment (i.e., thermal annealing) at a temperature of not lower than 800° C. in an As atmosphere, then the SiO2 film 1010 sucks up Ga atoms from the surface of the p-type AlGaAs first upper clad layer 1004a put in contact with the SiO2 film 1010, generating Ga holes in the p-type AlGaAs first upper clad layer 1004a. The holes diffuse until the holes reach the quantum well active layer 1003 inside the crystal, disordering the quantum well structure. The window region of the active layer in which the quantum well structure is disordered functions as a transparent window with respect to the oscillated laser light emitted in the internal region since the effective forbidden bandwidth of the active layer is extended.
Finally, the SiO2 film 1010 is removed, and the p-type AlGaAs-second upper Clad layer 1004b and the p-type GaAs contact layer 1005 are successively epitaxially grown on the p-type AlGaAs first upper clad layer 1004a (FIG. 18C). Next, a resist film is formed on the p-type GaAs contact layer 1005, and a stripe-shaped resist 1011 is formed in the same region as that of the stripe-shaped opening 1010a of the SiO2 film 1010 by the photolithographic technology. Next, a proton is injected from the surface side of the p-type GaAs contact layer 1005 with the stripe-shaped resist 1011 used as a mask, forming a high-resistance region 1007 that becomes a current block layer (current obstructing layer) (FIG. 18D). Finally, the n-side electrode 1008 is formed on the GaAs substrate 1001 side, and the p-side electrode 1009 is formed on the p-type GaAs contact layer 1005. The wafer is cleaved to obtain the semiconductor laser element of FIG. 17.
However, in the conventional window structure semiconductor laser element, the SiO2 film 1010 is formed on the surface of the p-type AlGaAs first upper clad layer 1004a so as to provide a bandgap larger than a bandgap corresponding to the laser oscillation wavelength in the disordered region formed in the vicinity of the resonator end surface, generating Ga holes in the p-type AlGaAs first upper clad layer 1004a put in contact with the SiO2 film 1010 and diffusing the Ga holes into the quantum well active layer 1003.
The generation and diffusion of the Ga holes occur in the region covered with the SiO2 film 1010. If heat treatment is performed at a temperature of not lower than 800° C., then Ga holes are generated though a little in amount by the re-evaporation of Ga atoms on the surface of the region (resonator internal region) that is not covered with the SiO2 film 1010, and the Ga holes diffuse into the quantum well active layer 1003. This consequently causes a reduction in long-term reliability due to the wavelength fluctuation accompanying the fluctuation in the bandgap of the quantum well active layer in the resonator internal region and due to the deterioration in the crystallinity of the quantum well active layer.
Moreover, the diffusion of Ga holes into the quantum well active layer 1003 in the resonator internal region can be restrained by lowering the heat treatment temperature or shortening the heat treatment time (i.e., annealing time). However, the generation of holes in the region covered with the SiO2 film 1010 and the diffusion of the holes into the quantum well active layer 1003 in the region located in the vicinity of the resonator end surface of the active layer under the region covered with the SiO2 film 1010 become insufficient, by which the laser light is disadvantageously absorbed in the window region of the active layer, or the region located in the vicinity of the resonator end surface of the active layer. As a result, the COD tends to easily occur in the active layer region located in the vicinity of the resonator end surface, causing a reduction in the maximum optical output when the element is driven at high power and failing in obtaining sufficient long-term reliability.